A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning structure, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.
The term “patterning structure” used herein should be broadly interpreted as referring to a structure that can be used to impart a projection beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the beam of radiation may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the beam of radiation will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
A patterning structure may be transmissive or reflective. Examples of patterning structures include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned.
The support structure supports, i.e. bares the weight of, the patterning structure. It holds the patterning structure in a way depending on the orientation of the patterning structure, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning structure is held in a vacuum environment. The support can be using mechanical clamping, vacuum, or other clamping techniques, for example electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable as required and which may ensure that the patterning structure is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning structure”.
The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “lens” herein may be considered as synonymous with the more general term “projection system”.
The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”.
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.
The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.
For the purpose of alignment, marker structures may be used which by their interaction with an impinging light beam (or alignment beam) generate one or more diffracted beams. From information collected from the one or more diffracted beams, the position of the marker relative to a position of the pattern on the mask can be determined. Typically, a marker consists of some periodic structure such as a grating, which includes a sequence of lines and trenches.
During the manufacturing process two scenarios may be used for the alignment of patterns at the gate level of a transistor.
Firstly, stand-alone alignment markers are exposed and subsequently etched in a zero layer (i.e., the base plane of a substrate) during a lithographic process. Alternatively, scribelane alignment markers are exposed and etched together with the first product layer, e.g. shallow trench isolation (STI). In the latter case, the processing of the zero layer is combined with a processing of a first layer.
The alignment marks basically follow the process flow of the product. After the etching, the trenches are filled with silicon dioxide (followed by a planarisation step). For a subsequent formation of, for example, a (MOSFET) transistor, a gate layer stack (for forming a gate) is deposited in blanket mode, also covering the marker structure of silicon dioxide filled trenches. The gate layer stack is either optically transparent or opaque to the radiation from the alignment beam. If the stack is transparent, then, the alignment beam can still reach the grating below the gate layer stack. In case the gate layer stack is opaque, no information can be obtained from the grating anymore.
Stand-alone zero layer markers are formed by an exposure step and a subsequent etching of the substrate top layer (i.e., silicon), before the actual process flow for creating a device structure such as an integrated circuit can be started. This pre-processing step of exposure and etching not only adds to the costs of manufacturing but it may lead to additional die losses and may adversely affect the yield on neighboring dies.
The use of scribelane markers may help in addressing issues associated with stand-alone markers in circumstances where the gate layer stack is transparent for the radiation of the alignment beam. In case the gate layer stack is opaque and the topology (i.e., the footprint of the grating structure on the surface of the gate layer stack) is negligible, a repair step becomes desirable to maintain alignment. Such a repair step may involve opening the gate layer stack to reach the original marker again. Moreover, a repair step adds further costs.
Also, the opacity of the gate layer stack may pose a problem for metrology tools in general.